At the present time there is strong demand for high-performance engineering materials lighter, stronger, and more elastic than existing ones for use in various sectors of industry including automobile, aircraft, and other diversified fields. Attracting attention because of its potential to meet this demand is carbon fiber, a formed carbon material.
Optically anisotropic pitches, one group of starting materials for carbon fiber manufacture, are described, for example, in the specifications of (Japanese, unless otherwise specified) Patent Application Public Disclosure Nos. 19127/1974, 89635/1975, and 118028/1975. In these pitches, most of the optically anisotropic phase (hereinafter referred to as "AP") portion is equal to a quinoline-insoluble (or pyridine-insoluble) portion. If the AP portion of such a pitch were close to 100% the softening point would rise sharply, necessitating a spinning temperature of nearly 400.degree. C. or even upward. Moreover, the spinning operation could induce generation of pitch decomposition gas and its polymerization. For this reason conventional spinning processes for the manufacture of carbon fiber limit the AP content to 90% or below, preferably in the range of 50 to 70%, and the spinning temperature to a level low enough to avoid appreciable thermal decomposition or polymerization.
Such a pitch composition of the prior art is a mixture of AP and a corresponding proportion of an optically isotropic phase (hereinafter referred to as "IP"), or a so-called heterogeneous pitch. The material therefore has shortcomings such as end breakages during spinning, ununiformity in fineness, or low strength of the resulting fiber.
Pat. App. Pub. Discl. No. 55625/1979 discloses a pitch material, homogeneous with 100% AP. The process for its preparation involves subjection of an isotropic pitch material to only a thermal reaction throughout, with careful control of thermal decomposition and polycondensation and uninterrupted agitation till attainment of homogeneity. After all, thorough thermal polymerization raises the softening point of the pitch material to about 300.degree. C. or above, requiring the use of a high spinning temperature in the vicinity of 400.degree. C., a level still too high for easy spinning operation on an industrial scale.
Other pitch materials have been introduced by the specifications of Pat. App. Pub. Discl. Nos. 160427/1979, 58287/1980, 130809/1980, 144087/1980, and 57881/1981. They are obtained by extracting an isotropic pitch or a pitch containing a very small proportion of AP with a solution, taking out the portion most of which forms AP and which has a low quinoline-insoluble content, and then melting that portion. It is true that a material with a specifically low quinoline-insoluble content of 25% or downward can be prepared by such a process. However, as will be readily appreciated by one skilled in the art from the process and pertinent data revealed, the product has such a high softening point that a high spinning temperature on the order of 400.degree. C. must be used, rendering it still difficult to carry out the spinning in a stable way industrially.
As stated above, the known homogeneous, optically anisotropic pitches with nearly 100% AP have so high softening points that they are difficult to spin stably. Low-softening-point pitches of the prior art, excepting those of unusual compositions and structures produced from special starting materials, are also difficult to spin because of the heterogeneity. Thus, obtaining a carbon fiber of excellent quality has been next to impossible.
As regards the conventional methods of defining the properties of pitch compositions, it is common practice to define an optically anisotropic pitch by its partial chemical structure, average molecular weight, or quinoline-insoluble (or pyridine-insoluble) content. These methods have, of course, been inappropriate because they cannot specify an optically anisotropic pitch composition with homogeneity and low softening point suitable for the manufacture of high-performance carbon fiber or other carbon products. This is explained from the fact that the composition known as optically anisotropic pitch is actually a mixture of very diverse, complex compounds with broad ranges of chemical structures and molecular weights, for example, from hundreds to tens of thousands, and in some cases short of the molecular weight of coke; the composition therefore cannot be simply defined by the characteristic chemical structure of a certain part or the whole on the average.
The present inventors have intensively studied on the optically anisotropic pitch composition suited for the manufacture of high-performance carbon fiber. As a result, it has already been found that the optically anisotropic pitch, a pitch with good molecular orientation having a developed structure of condensed polycyclic aromatics, actually contains various compounds, of which those having low softening points and suited for the carbon fiber manufacture have certain specific chemical structures and compositions. In other words, factors of great importance to an optically anisotropic pitch are the compositions, structures, and molecular weights of the n-heptane-soluble matter and the n-heptane-insoluble but benzene-soluble matter the particular pitch contains. The finding led to the filing of Pat. App. No. 162972/1980.
Continued research has been centered on the mixing ratio of AP and IP and the microscopic form of pitch. The research has revealed that, although a completely-single-phase pitch of substantially 100% AP having a softening point in the range of 250.degree. to 300.degree. C. can be made, the allowances for the conditions of producing such a pitch are rather limited and it is not always easy to produce industrially a stable pitch having a steady, sufficiently low softening point despite changes in material and which permits the use of the same proper spinning temperature.
On the other hand, a pitch containing the IP portion to excess, say 30% or upward, generally may have a satisfactorily low softening point. It has, however, been confirmed that the pitch during spinning behaves apparently as a mixed phase of two liquids differing in viscosity, exhibiting poor spinnability and hence yielding a carbon fiber of inadequate properties.
With the progress of research, a pitch has been unveiled which has an IP content of not more than about 20%, desirably not more than about 10%, most of the IP dispersed in the AP matrix being spheres having a diameter of about 100 .mu.m, preferably about 50 .mu.m or less, or more preferably in the form of microspheres having a diameter of about 20 .mu.m or less, and has a sufficiently low softening point. With good spinnability the pitch is an optimum precursor material for the manufacture of carbon fiber with adequate performance. As an additional advantage it has been found possible to produce the pitch stably with substantially unchanged properties in commercial operation. The findings materialized in Pat. App. No. 140782/1981.
The optically anisotropic pitch with the foregoing advantages is manufactured in a number of ways. Typically a carbonaceous pitch which partly contains AP in a molten state is held in a static condition at 350.degree. to 400.degree. C. where the thermal decomposition-polycondensation reaction does not proceed noticeably and most of AP easily settles down by gravity for unity, collecting the AP portion downward out of the pitch, and then separating and taking out the AP-rich lower layer from the overlying layer low in AP, as proposed by the present inventors. Patent Applications covering the method have been filed as Pat. App. Nos. 99646/1980, 162972/1980, 11124/1981, 135296/1981, and 140782/1981.
Thereafter, the present inventors have continued their search for a process whereby such an optically anisotropic pitch having desirable spinning properties, low softening point, and a high AP content would be produced in a more stable and economical manner. The research has led to the findings as below.
It has been found possible to produce an improved, excellent optically anisotropic pitch by the following method. A carbonaceous pitch partially containing AP is centrifuged in a molten state, so that the AP portion of heavier specific gravity is quickly settled and coalesced centrifugally, and the AP-rich half of the material is separated and taken out from the remainder of lighter specific gravity that consists mostly of IP.
Thus, in the field of centrifugal force exerted artificially the AP settling from the material pitch takes place far faster than in a static field of gravity provided the temperature is the same. Not only the AP settling but also the formation of a single phase due to the coalescence that follows the settling can be accelerated by the centrifugal action. It has further been found that, by freely controlling the centrifugal acceleration to be applied, or the rotational speed of the centrifugal operation, it is possible to produce an optically anisotropic pitch of a low softening point at an eventually adequate AP concentration and with good reproducibility, within a far shorter period of time, using the lower treating temperature, than by the gravitational settling or other prior art methods.
The newly found method has, however, a very high possibility of causing difficulties when fine solid particles are present in the carbonaceous pitch. First, the solids can invite end breakages and yarn unevenness during spinning. Second, they can find way into the resulting carbon fiber, causing a major structural defect or sharply reducing the tensile strength and breaking elongation of the fiber.
The expression "fine solid particles" as used herein means those originally contained in the raw material, for example, catalytic- or steam-cracking residue, liquefied coal, and the like; small to trace amounts of residual catalyst, rust, dirt, ashed granules, inorganic matter, and the substance detected as ash content; and carbonaceous solids or coky particles contained in the material; and those secondarily formed during pitch making or deposits falling off from the walls of the reaction vessel or piping. The mass of such fine solid particles (solid slurry) is observed on melt filtration or centrifuging as a quinoline-insoluble or high-C/H-ratio portion.